Picture this.  You are on the 2nd page of the structural drawings and come across the concrete mix designs.  12 mix designs are detailed in a nice little spreadsheet.  You read what MPa are required for each category but beyond that…you are not sure what the rest means.  KIM Mix? Air content? Is that F there because they already know I am failing any test on this?  If you have been there, don’t worry! You are not alone; I was too when I first started as a coordinator.   

When it comes to construction materials, few are as essential and versatile as concrete. This incredible substance serves as the foundation for countless structures, from residential homes to towering skyscrapers.  In 2022 alone, 4.1 billion tons were produced. (Source)  This makes it the second most consumed substance on Earth, after water. (Source) 

We all know the basics of concrete, but do you really understand what concrete mix is required for what situation? There is so much material out there on concrete that it may be overwhelming.  In this blog, we'll delve into what is in concrete, discussing its fundamental composition, exposure classes, standard additives, and touch on the main concrete tests you will see.  You will no longer wonder, what that F is for! Let's unravel the secrets behind this versatile and great material. 

Composition of Concrete: The Building Blocks 

Concrete at its roots is a basic mixture of 4 key components, carefully combined to achieve the desired strength and durability. The primary ingredients are:  

1. Cement 

2. Aggregates 

3. Water 

4. Air 

As a general, the ratio is 60-75% aggregate, 15% cement and water, and 5-8% air.  In addition to the key components, there are several additives on the market for a variety of situations.   

1. Cement: The Glue  

Cement acts as the glue that holds concrete together. It reacts with water to form a paste that surrounds and coats the aggregates. This paste gradually hardens, lending strength to the concrete over time. The strength of concrete is often measured in megapascals (MPa), indicating its ability to withstand compression forces – ie how much weight can be on it before it fractures/fails. Science Direct describes compressive strength as: “simply the applied load at failure divided by the cross-sectional area of the gauge-section (i.e. σ=P/A)” or simplified, “compressive strength can be defined as the capacity of concrete to withstand loads before failure.” (Source)  

Common concrete mixtures range from 20 to 40 MPa (after 28 days of curing), suitable for a wide range of applications. Though I have seen this as low as 5MPa, as high as 60MPa, and towers can have 70+MPa for the columns! 

2. Aggregates: The Backbone 

Aggregates are essential components in concrete, with two main types: coarse and fine. Coarse aggregates, consisting of robust particles like gravel or crushed stone, play a pivotal role in concrete's strength and stability. They provide the bulk, enhance load-bearing capacity, resist cracking, and promote sustainability through cost-effectiveness and drainage capabilities. The typical aggregate size is 20mm, with smaller sizes (commonly 10mm and 14mm) incurring premium costs. 

Fine aggregates, comprised of smaller particles like sand, contribute to the workability of concrete. They fill gaps between coarse particles, improving the mix's manageability, bonding with cement paste, and efficient use of cement. Fine aggregates are also influential in creating aesthetically pleasing finishes and maintaining consistency in architectural concrete. 

Balancing the use of both coarse and fine aggregates is crucial in concrete mix design. This balance ensures that concrete meets strength requirements while remaining workable during construction. In summary, coarse aggregates provide strength and stability, while fine aggregates enhance workability, durability, and aesthetic appeal in concrete mixtures. Finding the right balance between these two types is key to successful concrete construction. 

3. Water: The Hydration 

Water plays a crucial role in concrete, serving multiple functions in the mix. Firstly, it initiates the hydration process in cement, forming a strong crystalline structure and influencing concrete's final strength and durability. Secondly, water ensures the workability of the concrete, making it easy to place, consolidate, and finish while striking the right balance to prevent issues like segregation or excessive bleeding. 

Water also participates in chemical reactions when it interacts with certain additives or admixtures, such as superplasticizers, which can improve concrete's flowability without compromising its strength. 

Concrete mixtures are often defined by their water-cement ratio (W/C), representing the weight of water relative to the weight of cement in the mix. Lower W/C ratios generally yield stronger and more durable concrete but may reduce workability. Achieving the optimal balance between workability and strength is a critical consideration in concrete mix design, making water management a key aspect of successful concrete construction. 

4. Air Content: The Role of Air Entrainment 

Air entrainment is the intentional introduction of tiny air bubbles into the concrete mixture. These air bubbles enhance the concrete's durability by allowing slight expansion during freezing, reducing the risk of cracking. In regions prone to temperature fluctuations, air entrainment is a key consideration. Typically, you will see air content specified as 5-8%, but this can range from 1-8%.   

Now that you have gained insight into the fundamental building blocks, there are two important aspects to understand; Exposure Classes and Additives.  

Exposure Classes: Tailoring Concrete for Environments 

Concrete's durability is affected by the environment it's exposed to. Exposure classes categorize different conditions, such as freeze-thaw cycles or exposure to corrosive chemicals. Properly selecting an exposure class ensures that the concrete will perform optimally over time, reducing the need for frequent repairs. The main exposure classes are: 

• Freeze-thaw exposure (F). Examples – pool decks, patios, and exterior walls. 

• Sulfate exposure (S). Examples – concrete in contact with water or soil with high sulfate levels. 

• Chloride exposure (C). Examples – garage floors, porches, bridge decks, and underground slab on grades. 

• No chloride or freeze-thaw exposure (N). Examples – footings and interior slabs. 

• Agricultural, municipal, industrial projects exposed to human or animal waste (A). Examples – silos and bunkers, manure tanks, and sewage pipes. 

CSA A23.1 – Table 1 and 2 describe this in more detail. It can be seen at this link. https://www.atlanticconcrete.ca/images/Concrete-Exposure-Classes-2013_0.pdf 

Additives: Enhancing Concrete's Properties 

Standard additives, known as admixtures, are substances added to the concrete mixture to enhance specific properties. Here are some common types and their functions: 

Concrete Testing  

I will do a blog solely on concrete testing in the future, but for now I wanted to touch on two you will commonly see in construction.  These two tests serve the purpose of confirming the mixed concrete meets the design requirements.  

• Slump Test   

  • This test is performed on site with fresh concrete to measure the workability and consistency.   

  • Different construction applications will require different slumps. For example, a high slump is best for pumping concrete while a low slump is preferred for structural columns.  

• Compressive Strength Test 

  • This test is performed in a laboratory with a cylindrical sample taken from site to test the strength of the concrete. 

  • Different construction applications will require different strengths. For example, a sidewalk will require a lower strength while a building column will require higher.  

Conclusion 

Concrete is a remarkable construction material with a simple yet complex composition. The careful selection of aggregates, cement, exposure classes, air content, and additives contributes to its strength, durability, and suitability for diverse environments. By understanding these components and their roles, both contractors and homeowners can make informed decisions when embarking on construction projects. Whether you're building a foundation, a bridge, or a walkway, a solid grasp of concrete's fundamentals is the cornerstone of success.